US3424572A - Alloyed metallic powder process - Google Patents

Description

United States Patent 3,424,572 ALLOYED METALLIC POWDER PROCESS Niranjan M. Parikh, 412 Watseka St., Park Forest, Ill. 60466 No Drawing. Continuation-impart of application Ser. No.

244,887, Dec. 17, 1962. This application Sept. 13, 1966,

Ser. No. 578,990 US. Cl. 75.5 3 Claims Int. Cl. C22c 1/06; B22f [/00 This application is a continuation-in-part of my copending application, Ser. No. 244,887, filed Dec. 17, 1962, and now abandoned. This invention relates to a process for producing alloyed metal powders for use in powder metallurgy. It has particular reference to a process for producing low alloyed iron powder of a porous nature having large specific surface areas.

It is well known in the art that different properties of metals and metal objects can be improved by alloying with other metals. Generally, alloy powders have been made by melting the various metals together and subsequently disintegrating a molten stream of metal into powder form by means of a high energy air or water jet.

Another method of making alloy powders or compacts of alloy compositions has been a mechanical admixture of the alloying metals in metal powder form. Hitherto the results from using melting processes have been superior to the latter method. This is due to the fact that alloy powders or powder mixtures, so far available on the market, have not had sufficiently homogeneous composition, or if the grains were homogeneous, they did not have a spongy texture nor a large specific surface area. Both these qualities are highly desirable for a powder which is to be pressed and sintered into a strong metallic object having high green and sintered strength.

Three general methods have been in use for the manufacture of alloy powders to date. These are:

1) Mixing elemental iron powder with alloy additions in elemental form, e.g. adding nickel, molybdenum, manganese, chromium, etc.

(2) Mixing partially pre-alloyed powders, e.g. carburized and crushed steel powders and ferro-alloy powders.

(3) Producing powder from alloy steels, mostly by atomizing from the alloy melt.

The first two methods rely upon long sintering times and high sintering temperatures. In the case of mixed powders, compaction is greatly facilitated by the major proportion of soft iron, but the formation of a true alloy steel depends largely upon the extent of diffusion that can be obtained. In general such a sintering treatment calls for very high purity powders.

The sintering times and temperature could be reduced somewhat by using ferro-alloys, but this approach adversely affects the compactibility of the mixture. The higher concentration of ferro-alloys, the greater is the difficulty in compacting the powders. The lack of plasticity of the hard ferro-alloy powders prevents the formation of large contact areas in their immediate vicinity, resulting in weak compacts and poor sintering materials.

The currently used methods for production of atomized steel powders yield powders of relatively uniform composition, but such a powder requires very high molding pressures and sintering temperatures or hot-pressing operations. By proper manipulation of the atomizing medium it is possible to obtain non-spherical powder varieties, but none of these have the sponge-like structure so desirable in powder metallurgy applications.

Many attempts have been made in the past to produce alloy powders by co-reduction of oxides. But none of them has been too successful on a commercial scale. The

Patented Jan. 28, 1969 reason is that co-reduction of mechanically mixed oxide powders will invariably lead to selective faster reduction of one oxide than the other, thereby yielding a mixture of metal powders instead of an alloyed powder.

The principal object of this invention is to provide a new method of manufacturing alloyed powder, particularly a low alloy iron powder. This powder has a sufficiently homogeneous chemical composition to yield, under ordinary compression conditions and sintering times, a chemically homogeneous compact.

The powder also has a spongy texture, which is very desirable for obtaining high green and sintered strength compacts.

The diffusion rate of the oxides is much greater than the rate of diffusion of the metals. In other words, the oxides will homogenize faster than the pure metals.

The invention therefore consists in mixing reducible chemical compounds, e.g., oxides or other compounds of metals desired in the alloy, in calculated proportion to correspond to the alloy analysis, heating such a mixture to cause diffusion under non-reducing conditions and, after the desired diffusion is obtained, subject the mixture to a reduction, after which the reduced body can be comminuted to powder.

One of the objects of this invention is to provide an improved process for obtaining powdered alloys which after processing into compacts have superior resistance to fracture.

Another object of the invention is to produce a powdered alloy where alloy constituents have a more uniform and homogeneous distribution.

Another object of the invention is to homogenize all the constituents of a mixture of metal oxide powders to produce an alloy.

Referring now to the details of the present process, the invention includes the following steps. Predetermined concentrations of iron oxide (Fe O or Fe O nickel oxide (NiO), manganese dioxide (MnO or manganese oxide (MnO) and molybdenum trioxide (M00 are ground, mixed and compacted or extruded into compacts or pellets. Instead of compacting, the powders may be treated in uncompacted form but this requires longer treatment. These are then heated in oxygen, air, or other oxygen containing gas at an elevated temperature for an extended period; for example, one hour in air at 1,000 degrees centigrade. This step causes the different oxides to diffuse into each other and results in a material which is more homogeneous than can be produced by diffusion in the metallic state at comparable conditions.

The compacts are then reduced, either in their compacted form or after comminuting, in any commercial reducing atmosphere, at a pressure, time and temperature sufficient to yield a spongy, homogeneous alloy either in pellet, compact or powder form. For example, the compacts can be crushed to a powder and then heated to a temperature of 9001,l00 degrees centigrade in hydrogen for one hour at atmospheric pressure. However, the time, temperature, pressure and reduction media may be varied.

The process according to the invention can advantageously be carried out in a semi-continuous operation, e.g. by using a series of successive heating chambers or furnaces, comprising in the direction of flow of the powder mix first at least one preheating chamber operating under non-reducing, e.g. oxidation condition for causing the diffusion equalization, i.e., wherein only one solid phase is present in the composition, and then a reducing chamber followed by one or more cooling chambers for decreasing the temperature of the reduced mix to such a low temperature that reoxidation is substantially obviated when exposing the reduced mix to the air.

Such continuous operation can also be carried out in one and the same furnace, e.g. in a tunnel kiln having a central high temperature reduction zone with decreasing temperature in both directions from the same to form preheating and cooling zones respectively. By suitably dimensioning the length of these zones in relation to one another, it is possible with -a constant rate of forward travel for the material through the kiln to obtain the desired homogenization or diffusion equalization and reduction in a fully continuous operation.

When using a series of successive heating chambers or furnaces it is easy to maintain the different non-reducing or oxidizing conditions and the reducing conditions in the different chambers. Also in one and the same relatively long furnace, e.g. a tunnel kiln, it is known per se how different reducing to oxidizing conditions can be maintained in different zones by using dampers and pressure regulation. When using a tunnel kiln the most simple measure is to mingle the starting powder mixture with or to embed it in a solid reducing agent such as coke or charcoal since such agents do not have a reducing action on iron oxide or the like until the temperature has attained about 900 C. whereas diffusion occurs already at a considerably lower tempera-ture.

The reduced alloy depending on reduction time and temperature may have to be comminuted again to powder form, and may now be used directly in powder metallurgy applications.

The following oxides and their mixtures can be used in the above mentioned method in varying degrees: M1102, MnO, Fe O F6304, FCO, COO, M003, W PbO, CuO, SnO 'Ag O, C11 0, and CdO. Other compounds of these metals may be used provided they are reducible by hydrogen or any other commercial reducing atmosphere.

Example Nickel percent 1.5 Manganese percent 0.5 Molybdenum percent 0.2 Iron Balance The following materials were mixed, after being ground to a fine powder:

Grams Iron oxide (Fe O 134.2 Manganese dioxide (MnO 0.79 Molybdenum trioxide (M00 0.3 Nickel oxide (NiO) 191 The particle size is not too important but as in most powder metallurgy application's about 100 mesh may be used.

The above materials were blended by shaking in a glass jar to obtain a homogeneous mixture and then the mixture was formed into small cylindrical pellets x inch) without any binder but with considerable pressure. The pellets were then placed on a steel plate and heated at 1,000 degrees centigrade for on hour in air. This heat treatment gives the proper molecular diffusion of the oxides which provides the homogeneity necessary for the new result produced by this process.

The homogenized pellets were then crushed to a coarse powder and placed in a steel boat in a mufiie furnace. The temperature was then raised to 1,000 degrees centigrade while hydrogen was passed through the furnace. After one hour the powder was completely reduced to the metallic state. The xnufiie was then cooled to room temperature. A chemical analysis of the powder was as follows (by weight):

Nickel percent 1.5 Manganese percent 0.48 Molybdenum percent 0.2 Iron Balance The powder was then crushed to a 65 mesh screen size and mixed with 0.9 percent graphite. Transverse rupture test bars were then prepared by cold pressing the mixture in a steel die to a density of 6.1 grams per cubic centimeter. These bars were then sintered at 1,120 degrees centigrade for one-half hour in an atmosphere of hydrogen, cooled to 840 degrees centigrade, quenched in oil, and then tempered at 205 degrees centigrade for one hour. The resulting bars had transverse rupture strengths of 150,000 to 160,000 pounds per square inch with a Rockwell C hardness of about 30. When the same powder was mixed with graphite and compacted to a density of 6.5 grams per cubic centimeter and then heat-treated as described above, the transverse rupture strength was 190,000 to 200,000 pounds per square inch. These values are considerably higher than those obtainable from powders consisting of mechanical mixtures of the metallic ingredients.

It should be noted that the powder particles contain all the alloy elements in a highly homogeneous mixture rather than showing poor diffusion of the elements.

The reference to the formation of the powder mixtures into pellets before applying heat, as used in the specification and in the appended claims is not to be taken in any limiting sense since these compacted masses may be formed simply by extrusion means and in some instances heat may be applied to the uncompacted mixtures.

While the above described process included, by way of examples, specific temperatures and lengths of treatment, the invention is not restricted to these specific values. It has been found that the same results can be obtained by diffusing the mixture of metal oxides at 1,000 degrees for several hours or, the temperature can be increased to 1,200 degrees for less than one hour. Time and temperature are, of course, correlative factors and the higher the temperature, the shorter the treatment time required. The only limitations of the process are to be determined from the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process of preparing a spongy iron of low alloy content comprising the steps of mixing finely comminuted iron oxide with the oxides of the other metals desired in the alloy, subjecting the mixture to a diffusing heat treatment in an elongated furnace under non-reducing conditions at a temperature of between 980 and 1200 C. for about one hour to cause a homogenization of the composition of the mixture by diffusion wherein only one solid phase is present, and then subjecting the thus heat treated mixture to a reduction at a temperature below the melting point of the starting materials and of the homogenized oxide material and finally grinding said material.

2. A process as claimed in claim 1 for the production of iron powder alloyed with 0.1 up to 5% of at least one of a metal selected from the class consisting of nickel, molybdenum, manganese, in which a finely comminuted oxidic iron ore is mixed with finely comminuted oxide of at least one of the metals nickel molybdenum and manganese in proper amounts, said mixture being agglomerated and said agglyomerates being subjected to a diffusion treatment within the temperature range of 700-1200 C. for a period of about one hour, after which the agglomerates are crushed to pow- 5 der and the powder reduced with a gaseous reducing agent at a temperature range of 800-1'200 C.

3. A process as claimed in claim 2 for the production of an alloy powder of the type 4600 inwhich about 134 parts of lfinely comminuted magnetite concentrate is mixed with 08 part of manganese oxide, 0.3 part of molybdenum trioxide, and 1.9 parts of nickel oxide and thoroughly blended, the mixture formed into small pellets by compression without the use of any binder, said pellets then being heated in air for 1 hour at a temperature of 1000 C., cooled, crushed again to powder and reduced with hydrogen at a temperature of 900- 1000 C. and after reduction cooled in hydrogen atmosphere to avoid reoxidation.

References Cited OTHER REFERENCES Fundamental Principles of Powder Metallurgy, Jones,

10 W. D.; Pub. 1960, pp. 703-708.

HYLAND BIZOT, Primary Examiner.

W. W. STA'L'LARD, Assistant Examiner.

Claims (1)

1. A PROCESS OF PREPARING A SPONGY IRON OF LOW ALLOY CONTENT COMPRISING THE STEPS OF MIXING FINELY COMMINUTED IRON OXIDE WITH THE OXIDES OF THE OTHER METALS DESIRED IN THE ALLOY, SUBJECTING THE MIXTURE TO A DIFFUSING HEAT TREATMENT IN AN ELONGATED FURNACE UNDER NON-REDUCING CONDITION AT A TEMPERATURE OF BETWEEN 980* AND 1200* C. FOR ABOUT ONE HOUR TO CAUSE A HOMOGENIZATION OF THE COMPOSITION OF THE MIXTURE BY DIFFUSION WHEREIN ONLY ONE SOLID PHASE IS PRESENT, AND THEN SUBJECTING THE THUS HEAT TREATED MIXTURE TO A REDUCTION AT A TEMPERATURE BELOW THE MELTING POINT OF THE STARTING MATERIALS AND OF THE HOMOGENIZED OXIDE MATERIAL AND FINALLY GRINDING SAID MATERIAL